Specifying `RestrictChildren` and empty `HtmlTargetElement` results in an error

```
[RestrictChildren("div")]
[HtmlTargetElement]
public class MyTagHelper : TagHelper
{

}
```

``` HTML
<p>
    <my>
        <div></div>
    </my>
</p>
```

Expected: This should render MyTagHelper correctly
Actual: `The <my> tag is not allowed by parent <p> tag helper. Only child tags with name(s) 'div' are allowed.`